DOCS. 59, 60 MARCH 1915 71 could be taken instead; because if the mass is critical at the developmental stage, then for binary stars the mass of the components is much more critical than the overall mass. 5) You should provide me with a bit more precise information on how the radii of the eclipses are estimated.[6] 6) Is the absence of the effect really proven for types that yield smaller masses using the binary-star method? Why do you pass over this important question so quickly?[7] 7) Did you keep yourself open to the hypothesis that for stars with only one visible component, both components have masses in the same order of magnitude? It is a shame that your descriptions are not detailed enough to be able to es- timate the uncertainty attached to your estimates. Thus a nonspecialist cannot get a notion of the reliability of your calculations. A much more in-depth pre- sentation would be desirable. The worst in this regard is the specification of the mean densities. In the formula in the fourth column of the proofs, “V0.1” is inadvertently indicated[8] instead of “3V0.1.” With best regards, yours, Einstein. 60. To Tullio Levi-Civita [Berlin,] 5 March 1915 Highly esteemed Colleague,[1] By examining my paper so carefully, you are doing me a great favor.[2] You can imagine how rarely someone delves independently and critically into this subject. I also cannot help admiring the uncommon sureness with which you make use of a language that is foreign to you. When I saw that you are directing your attack against the theory’s most important proof, which I had won by the sweat of my brow, I was not a little alarmed, especially since I know that you have a much better command of these mathematical matters than I. Nevertheless, upon thorough consideration I do believe I can uphold my proof.[3] I start with the second part of your letter in which you intend to show with an example that the result of §14, that Euv/-g is a tensor, is not correct. For this